The present invention relates to the field of display controllers. More particularly, the present invention relates to set-up and synchronization of a display controller for receiving analog video signals from a host computer system and for converting the analog video signals into digital samples prior to provision to a display monitor. The present invention also relates to communication of information from the host computer system to the display controller.
A desk top personal computer system typically includes a video adapter card connected to a system bus for the computer system along with a central processing unit (CPU) and memory. Data representative of an image to be displayed is typically generated by an application program stored in the memory and executed by the CPU. The image data is typically provided to the video adapter card via the system bus. A cathode ray tube (CRT) display monitor is typically connected to the video adapter card for displaying the image. Accordingly, the video adapter card forms signals which are appropriate for driving the CRT display.
An image is formed on the CRT by directing an electron beam, which originates from behind the display area of the CRT, according to a repeating pattern of equally-spaced horizontal scan lines which cover the display area of the CRT. This pattern is known as a scanning raster. One complete pass over the display area is referred to as a frame. For driving the CRT display, the video adapter card generates red (R), green (G), and blue (B) component signals along with a horizontal sync (HSYNC) signal and a vertical sync (VSYNC) signal. The RGB signals provide color and intensity information for the raster, while the HSYNC signal provides timing information for decoding horizontal scan lines for the scanning raster. The VSYNC signal provides timing information for decoding vertical retrace periods for the raster which occur between frames. The vertical retrace periods are accompanied by a vertical blanking interval during which the RGB signals do not convey color information (e.g., they are held at their lowest intensity level), while horizontal retrace periods are accompanied by horizontal blanking intervals. A portion of the horizontal blanking interval prior to a pulse in the HSYNC signal is known as a front porch, while a portion of the horizontal blanking interval following a pulse in the HSYNC signal is known as a back porch. Phosphor coatings on the CRT convert the electron beam into red, green and blue visible light, thus, forming a color image on the CRT.
The RGB, HSYNC and VSYNC video signals are analog signals which are typically formed in accordance with one of a variety of standardized formats. These standardized formats include: Enhanced Graphics Adapter (EGA), Video Graphics Array (VGA), Super VGA, Extended Graphics Array (XGA), and Color Graphics Adapter (CGA). Each standard specifies available display modes, color resolutions, spatial resolutions, character sets, available graphics functions and a variety of other display parameters.
The spatial resolution in the vertical direction for the analog video signals is related to the number of horizontal lines which occur between vertical retrace periods. The spatial resolution in the horizontal direction is related to a number of cycles of a clock signal which occur during each horizontal line. A clock signal (e.g., a pixel clock) is formed by the computer system and is utilized for forming the analog video signals. For each cycle of the clock signal, the RGB analog video signals include constituents, such as color and intensity information, for display. Accordingly, each such clock cycle represents the minimum size for a feature of an image which may be displayed. Although the analog video signals may not literally have pixels, each such clock cycle which occurs during a horizontal scan line may be thought of as corresponding to a pixel. Thus, each horizontal scan line includes a row of pixels.
Digital displays, such as flat panel display (FPD) monitors, have been used for some time in laptop and notebook portable computers, and have become increasingly prevalent in desktop computer systems due to several advantages that FPD monitors offer over CRT displays. For example, in comparison to a CRT having the same display area, an FPD monitor is generally lighter in weight, occupies less space and consumes less power than the CRT counterpart display.
An FPD monitor typically includes liquid crystal sandwiched between two layers of polarized material. The alignment of crystals for an area of the display, such as a pixel, can be controlled by applying an electric field to the area. The electric field for each pixel can be controlled by applying voltage to the area via a transistor switch associated with the pixel. Due to the polarization of the layers, the alignment of the crystals for the area also affects the ability of the area to transmit light. Thus, by providing a backlight for the display and by activating appropriate ones of a matrix of transistor switches, an image can be formed on the display. By providing colored filters and additional transistor switches for each pixel, such that a red, green and blue display element is provided for each pixel, color images can be displayed by the FPD.
Because individual display elements of an FPD monitor must be appropriately controlled to form an image, the character of signals required to drive FPD monitors differ markedly from those required to drive CRT displays. More particularly, FPD monitors are generally driven by digital signals in accordance with an appropriate digital protocol, whereas, the RGB, HSYNC and VSYNC video signals formed by the video adapter card are in accordance with an appropriate analog format, as explained above. This has typically required different adapter cards for CRT and FPD monitors.
To drive an FPD monitor using a computer system which is preconfigured for driving a CRT, the analog RGB, HSYNC and VSYNC video signals must be transformed into appropriate digital signals. In addition, it may be desired to perform digital processing on the analog video signals prior to providing such digitally-processed signals to a display monitor. For example, the digitally-processed signals may be provided to a digital display, such as an FPD, or may be converted back into analog signals prior to provision to an analog display, such as a CRT. In either case, the analog video signals must be digitally sampled. The sampling frequency and phase, however, must be precisely synchronized with the analog video signals. Otherwise, the display image can be degraded on the whole, can include areas that are blurred, or can be misaligned to the display area of the display monitor. The synchronization task is further complicated in that application programs often alter the parameters of an analog video signal from those parameters specified by one of the standardized formats (e.g., SVGA).
Therefore, what is needed is are improved techniques for converting analog video signals into digital samples for driving a display monitor. More particularly, what is needed are improved techniques for synchronizing a digital sampling frequency and phase with that of an analog video signal; for horizontal and vertical orienting a display image on a display monitor; and for communication from a host computer system to a display controller. It is to these ends that the present invention is directed.